1. Field of the Invention
This invention relates to laminated matrix composites, a new class of materials made with a CVI process comprising a reinforcement preform structure coated with multiple thin layers of a matrix material
2. Description of the Prior Art
It is well known that ceramics have, for certain applications, desirable properties, such as light weight, high stress, corrosion/wear resistance, and strength retention at high temperatures. In recent years, ceramics have been the focus of considerable attention for use in advanced energy conversion systems such as heat exchangers, gas turbines, and other heat engines. However, their brittleness limits their use in most structural applications. Metals, on so the other hand, have excellent toughness, but typical suffer from loss of strength at high temperatures, excessive creep, and high density. These shortcomings have been overcome for ceramics and metals using fibers or whiskers as reinforcement and also in metals using platelets and particulates. For example, the toughness of SiC and carbon have been improved by reinforcement with SiC and carbon fibers. Also, SiC fibers or platelets have been used to reinforce Ti, Al and other metals. In these prior examples, the matrix was either single phase or contained a dispersed phase.
It is also well known that the mechanical properties of structures can be enhanced by using alternate layers of two materials. Examples of such laminated materials include Ni/Cu, Fe/Cu, ZrO.sub.2 /Al.sub.2 O.sub.3, SiC/C, TiC/TN, TiC/TiB.sub.2, TiC/Ni Al.sub.2 O.sub.3 /Nb, and many others. Much of the work conducted on such structures shows that mechanical and tribiological properties improve as layer thicknesses decrease. For some systems, properties show non-linear changes as layer thicknesses approach .about.0.02 .mu.m, i.e.; even more rapid improvement in properties with decreasing thickness. The high fracture toughness of mollusk shells (.about.10 Mpa.cndot.m1/2) is sometimes offered as an example of the potential of layered structures.
The approach of the prior work involving ceramic composites consisted of interspersing thick layers of ceramic with thin, less rigid layers of an interface material The vast majority of the matrix consisted of one phase, say SiC, which was partitioned into 3 to 5 thick layers by thin layers of an interface material such as carbon or BN. In these composites, the choice of matrix materials is limited to rigid ceramics and less rigid interface materials. Further, the layers of the ceramic material of these composites are in the order of several microns thick.
For example, U.S. Pat. No. 5,079,039, issued to Heraud. et al discloses a fiber preform densified by sequential depositions of thick layers of a rigid ceramic material interspersed with thin layers of either pyrolytic carbon or boron nitride. While some layers less than 2 .mu.m thick are taught in Heraud, et al., the alternating layers are magnitudes greater in thickness, thereby yielding an average thickness which is magnitudes greater than 2 .mu.m U.S. Pat. No. 5,246,736, issued to Goujard et al, discloses a similar composite coated with an oxidation-resistant coating of an Si-B-C system
Additionally, many of the types of fibers used as structures reinforcement are easily damaged by high temperatures. Thus, the use of fibers as the reinforcement structure in the prior art has limited the processing temperatures of the coating process. The use of less temperature sensitive reinforcement structures would allow higher processing temperatures, and thus a more rapid process and a more economical composite. Further, several materials require higher processing temperatures and thus are not suitable as matrix materials.
Further, HCl which may be present in a CVD atmosphere reacts with some fiber reinforcement materials and degrades them just as high temperature would. Thus, the ability to use less-sensitive structures, such as platelets or particles, would be a significant advance in the art.
Chemical vapor deposition (CVD) is one of the most widely used deposition processes to coat surfaces. The conventional CVD process is based on thermochemical reactions such as thermal decomposition, chemical reduction, displacement and disproportionation reactions. CVD reaction products find applications in a wide variety of fields, such as providing hard coatings on cutting tools, protecting surfaces against wear, erosion, corrosion, high temperature oxdation, high temperature diffusion, solid state electronic devices, preparation of fibers for composite materials, and hermetic coatings.
Chemical vapor infiltration (CVI) is a specialized form of CVD. In the CVI process, a matrix is chemically vapor deposited wit a porous preform to produce a composite material. CVD, in general, results in the production of a coating, while CVI results in the production of a composite article. The preform, or reinforcement phase, may consist of particulates, fibers, or any other suitable constituents or materials which will form a porous medium The preform to be subjected to CVI is placed in a modified CVD reactor. Gaseous CVD reagents penetrate the pores of the preform and deposit onto the surfaces of the particles. As the deposition process continues, the particles are coated and grow, and consequently the spaces between the particles become smaller. Eventually, the particle coatings interlock and the particles are bonded together by the coating. This coating is the matrix, which, along with the original particles, constitutes the composite.
It would be a significant advance in the art to combine the advantages of fiber or particulate reinforcement and laminated structures. The resulting composite would have a reinforcement phase and a laminate structure. Laminated structures are typically fabricated by stacking foils, followed by hot pressing or diffusion bonding, various coating processing, sedimentation, centrifugation, and electrophoresis. These processes, however, do not lend themselves to the infiltration of fibrous or particulate preforms. Furthermore, several of the processes now known are not applicable to submicron thick layers because of difficulties with handling or limitations on the size of the constituents.